Give it time. The commerce servers are not down, they are merely incredibly busy with all the people trying to access them. The result is that they are running pretty slowly at the moment.

If it says "Starting secure connection..." then it's fine... it is trying to connect. This may take minutes as opposed to seconds. If successful you'll get the option to purchase. If unsuccessful it will say "Failed to connect...", at which point you should restart the launcher and try again.